Utilizing a detector to measure the energy spectrum of high energy rays such as X rays or γ rays is one of important means for nuclide recognition. Such a detector has been widely applied in the fields of nuclear radiation protection, nuclear security inspection, environmental protection and homeland security, etc. for detecting radioactive substances. In the prior art, such a detector is mainly divided into two classes: one is a scintillator detector with NaI (Tl) as a representative, and the other is a semiconductor detector with high purity germanium (HPGe) as a representative.
The scintillator detector has the advantages of simple manufacturing and low price. A portable γ spectrometer used in the detection site is usually a NaI or CsI scintillator detector. However, the energy resolution of the scintillator detector is poor, with an energy resolution of 6%-7%@662 keV, which cannot meet the measurement requirement for the fine structure of a complex energy spectrum.
The energy resolution of the high purity germanium semiconductor detector is higher than that of the scintillator detector. However, the high purity germanium semiconductor detector can only be preserved and used at liquid nitrogen temperature (77 K), and cannot be used at room temperature. On the one hand, it is necessary for the high purity germanium semiconductor detector to be equipped with a cryogenic container and a vacuum chamber, which results in its increased volume and cost. On the other hand, when the high purity germanium semiconductor detector is used, it is necessary to frequently add liquid nitrogen, causing it to be unable to meet the requirement for use in field detection site, and its range of usage is limited.
In recent years, another semiconductor detector has appeared which can work at room temperature, and such a semiconductor detector uses a semiconductor crystal whose material is HgI2, GaAs, TiBr, CdTe, CdZnTe (Cadmium zinc telluride, abbreviated as CZT), CdSe, GaP, HgS, PbI2, or AlSb. Such a semiconductor detector has the advantages of a small volume, being easy to carry, high energy resolution, high detection efficiency and being capable of working at room temperature. Currently, such a semiconductor detector has been widely applied in the fields of environmental monitoring, nuclear medicine, industrial non-destructive detection, security inspection, nuclear weapon penetration, aeronautics and astronautics, astrophysics and high energy physics, etc.
The forbidden band of the CdZnTe semiconductor crystal is 1.57 eV, its impedance is as high as 1010 Ω/cm, its average atomic number is 49.1, its density is 5.78 g/cm3, the energy needed for generating one electron-hole pair is 4.64 eV, and it is the only semiconductor material which can work at room temperature and deal with 2 million photons/(s·mm2). Studies have shown that, a semiconductor detector using CdZnTe semiconductor crystals has the best performance and is most suitable for use at room temperature.
As compared to the scintillator detector, the energy resolution of the CdZnTe detector is improved, and its energy resolution is evidently higher than that of the NaI scintillator detector. As compared to the HPGe detector, the forbidden band of the CdZnTe detector is broader, its impedance is larger, its carrier concentration is lower, which makes its dark current smaller after a bias voltage is applied, and it is a semiconductor detector which can work at room temperature.
However, the CdZnTe crystal is generally inhomogeneous, and there are structural defects in it, therefore, the carrier mobility of the CdZnTe crystal is low, the carrier drift time is long, and the carrier (especially the hole) trapping phenomenon is easily produced, namely, the carrier lifetime is short. The carrier trapping phenomenon results in that the energy resolution of the CdZnTe semiconductor detector is reduced, and there occurs a low-energy tail phenomenon in the energy spectrum obtained by measuring by employing the CdZnTe semiconductor detector.
To improve the energy resolution of the CdZnTe semiconductor detector, the CdZnTe semiconductor detector generally employs an electrode having a unipolar charge sensitive characteristic. Such an electrode forms an electric field, and electrons and holes generated through interaction between high energy rays and the crystal move in different directions under the effect of the electric field, wherein the electrons move towards an anode, and the holes move towards a cathode. Since the weighting potential at a position far away from a collecting electrode is very small, the contribution to an induced signal from the movement of the holes at a position far away from the collecting electrode is quite small and the induced signal is mainly contributed by the electrons, thereby realizing a unipolar charge sensitive semiconductor detector. In the prior art, a CdZnTe semiconductor detector based on the unipolar charge sensitive characteristic mainly comprises the following types: Parallel Frisch Grid, Coplanar Frisch Grid, Hemisphere, CAPture, Quasi-hemisphere, and Pixelated, etc.
The unipolar charge sensitive semiconductor detector may reduce to a certain extent the adverse effect on the energy resolution due to a low migration rate and a short lifetime of a hole. However, a moving electron will also be trapped under the effect of the defect of the CdZnTe semiconductor crystal, and especially in the case of the electric field intensity being weak and the drift time being long, it is significant that electrons are trapped, which results in a fluctuation of the amplitude of the output signal of the collecting electrode of the CdZnTe semiconductor detector, thereby affecting the energy resolution of the CdZnTe semiconductor detector.
From the above, there is a need for further improving the energy resolution of the CdZnTe semiconductor detector.